This invention relates generally to an apparatus and method for controlling a variable displacement hydraulic pump and, more particularly, to an apparatus and method for controlling variations in pump discharge pressure caused by load variations.
Variable displacement hydraulic pumps, such as axial piston variable displacement pumps, are widely used in hydraulic systems to provide pressurized hydraulic fluid for various applications. For example, hydraulic earthworking and construction machines, e.g., excavators, dozers, loaders, and the like, rely heavily on hydraulic systems to operate, and hence often use variable displacement hydraulic pumps to provide the needed pressurized fluid.
These pumps are driven by a constant speed mechanical shaft, for example by an engine, and the discharge flow rate, and hence pressure, is regulated by controlling the angle of a swashplate pivotally mounted to the pump.
Ideally, it is desired to maintain a desired output pressure, i.e., the pump discharge pressure, for a given swashplate angle. However, variations in loading on the hydraulic system may require the pump discharge pressure to be varied as well, which in turn requires changes to be made to the angle of the swashplate. These changes, in conventional pump control systems, often result in overshoot, i.e., pressure spikes. Thus, relief valves must be used to prevent these pressure spikes from damaging the pump or hydraulic system.
In many conventional design pump systems, the pump discharge pressure is fed back to a biasing servo, which is configured to increase the swashplate angle as the pump discharge pressure increases. The increased swashplate angle further increases the pump discharge pressure, thus leading to an unstable open loop condition of the pump.
It is desired to develop a control system for a variable displacement pump which utilizes the benefits and simplicity of a linear first order dynamic system which eliminates overshoot, thus eliminating the need for relief valves. To accomplish this, it is also desired to configure the variable displacement pump so that the open loop system is internally stable.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention an apparatus for controlling a discharge pressure of a variable displacement hydraulic pump is disclosed. The apparatus includes a swashplate pivotally attached to the pump, a valve plate located on the pump to allow hydraulic fluid to enter the pump through an intake port on the valve plate, and to exit the pump through a discharge port on the valve plate, the hydraulic fluid entering and exiting the pump responsively creating a pressure carry over angle xcex3, a control servo operable to control an angle of the swashplate relative to the pump, a servo valve having an output port hydraulically connected to the control servo and an input port hydraulically connected to the pump output port, and means for controlling the servo valve as a function of the discharge pressure of the pump and responsively balancing a torque induced by the pressure carry over angle xcex3 with a torque generated by a control pressure Pc at the control servo.
In another aspect of the present invention a method for controlling a discharge pressure of a variable displacement hydraulic pump is disclosed. The method includes the steps of sensing a level of the discharge pressure at the pump output port, diverting a portion of the pump discharge pressure to a servo valve, delivering a control signal to the servo valve as a function of the sensed level of discharge pressure, and delivering a responsive hydraulic control flow from the servo valve to a control servo, the control servo being operable to control an angle of the swashplate, the hydraulic control flow from the servo valve providing a control pressure Pc at the control servo operable to balance a torque induced by a pressure carry over angle xcex3 of a valve plate located on the pump.